A. Field of the Invention
The present invention relates to a conductor paste for plugging a through-hole formed to connect wiring circuits formed on the opposing faces of a ceramic substrate. This invention also relates to a ceramic circuit board which incorporates this paste. More particularly, this invention relates to a conductor paste for plugging a through-hole in a ceramic single layer substrate, for plugging a through-hole in a ceramic multi-layer substrate, or for plugging a through-hole formed in an IC or transistor package substrate, as well as the ceramic circuit boards which incorporate this paste.
B. Description of the Related Art
In general, a ceramic circuit board is formed by printing a paste containing an electrically conductive powder, such as Ag powder, a mixture of Ag and Pd powders, a mixture of Ag and Pt powders, or Cu powder, onto a ceramic substrate, typically containing 96% alumina, by screen printing the paste onto the substrate and then sintering both at around 800-900.degree. C.
In recent years, with the advances which have been made in the miniaturization and performance levels of electronic devices, a demand has arisen for multi-layered ceramic circuit boards which are highly reliable and on which electronic parts can be installed at a higher density. A common means for increasing circuit density is to form wiring circuits on both faces of the substrate, with a through-hole being formed in the substrate to connect the wiring circuits on the opposing faces.
One method of connecting circuits using a through-hole is to print a conductor onto the wall face of the through-hole, while another method is to plug the through-hole with a conductor plug. With the plugging method, the conductor plug generally consists of a conductor paste containing an electrically conducting powder, which is inserted into the through-hole and sintered.
However, the plugging method is often inadequate in that the conductor plug tends to shrink and fall out of the through-hole after sintering due to the fact that the shrinkage force of the electrically conductive powder exceeds the adhesion strength between the conductor plug and the ceramic substrate. Another problem that plagues the plugging method is that the conductor plug tends to chip during the leveling process, i.e., the process of removing protruding portions of the conductor plug by grinding. Protrusions often occur after the plugging process because in order to plug the conductor paste into a through-hole with a diameter of 0.3 mm, it is necessary to use a printing plate which has a pat-pattern of a diameter of about 0.5 mm to accommodate the dimensional tolerance of the substrate to be printed. Consequently, conventional methods for connecting circuits via a through-hole have been practically limited to printing a conductor onto the wall face of the through-hole.
The applicant has already suggested a low shrinkage conductor paste for plugging through-holes which has rhodium powder incorporated into it (Japanese Patent Application No. 6-51306). The low shrinkability of this paste prevents the conductor plug from falling out of the substrate and has contributed to the improvement in wiring circuit density on ceramic substrates, but the conductor plug still suffers from the problem of chipping during the leveling process.
Conductor pastes are also used in the field of package substrates, which are another application for ceramic circuit boards. These substrates may be produced using multi-layer substrate technology in which green (unsintered) sheets of about 92% alumina are laminated together and sintered at a high temperature. A package substrate, an example of which is shown in FIG. 1, has an active part 7, such as IC or a quartz oscillator, mounted on a substrate 3, with the active part 7 being covered by a metal cap 6. Through-holes 2 are formed in the substrate 3 and a conductor plug 1 is plugged into the holes connecting the active part 7 to an environment outside of the metal cap 6. This serves to release heat which builds up within the metal cap 6. An inactive gas is sealed within the metal cap 6 to prevent deterioration of the active part 7. Hence, it is important that the seal between the cap and the substrate have a very low level of gas permeability.
When multi-layered package substrates are made from 92% alumina, gas permeability is maintained at a very low level by the fact that the package is multi-layered. Substrates made from 96% alumina have a very low level of gas permeability even when they are single-layered. When a through-hole in a 92% alumina multi-layered package substrate is to be plugged, the conductor paste and the substrate itself are sintered at the same time. In this case, a very low level of gas permeability in the through-hole is obtained only if the amount of shrinkage in the conductor plug and the substrate are matched. When a package board or the like is produced from a 96% alumina substrate, the through-hole is plugged with an unsintered conductor paste after the substrate has been sintered. When the package is resintered, a small gap between the through-hole wall face and the conductor plug will often form, even when a conductor paste that has low shrinkage is used, therefore making it difficult for the through-hole to have a very low level of gas permeability.